Platy fish with red dot

Question
Hi there, over a year ago I had a ten gallon set up with 5 (sun burst) platy fish in it. One had a red dot on her tail and I didn't really think much of it, but in the last couple months it's spread and developed into a white lump(s?) It appears as though the skins almost bubbled and in the past month or so it's started to deteriorate the bottom half of her tail fin. I've been dosing with melafix and pimafix for over a month now, I think I've tried every fungus med on the market. I have no idea what it is, I've looked it up and anything I find that sounds similar says that the disease is highly contagious (like wipes out entire lakes). This platy has lived with others for it's whole life, and she is the only one. Until a few weeks ago I noticed a tiny red dot starting on a younger platy's tail. So I've talked with someone and they recommended using tetracycline... I have people tetracycline, can I use that? Whats the dosage? I'm unable to set up a QT right now as my tanks are at my parents house and I don't live there. My mom would FREAK if she came home and I set up a tank. So would it be safe to treat the whole tank? I kind of want too now that a second fish is showing symptoms (Her dot is on her tail fin as well).

Thanks for reading, do you have any experience with this mysterious red dot/ white lump thing? I'm really at a loss of what to do, it appears the fish is healthy besides the obvious nastiness on her tail, and her growth is definitely stunted. But she swims around fine, shes got a voracious appetite, I would feel bad having to euthanize her when I haven't done everything possible to save her. Please help!

Answer
Hi,
Tetracycline is what I would use. I use person tetracycline (because its the same as fish tetracycline) but it has to be pure, with nothing else in it. I haven't had to do it in so long, I honestly forget the dosage. Ask someone at your pet store, they may be able to help with dosage. be sure to crush it into powder before adding it to the tank.